Electrolytic cell including titanium hydride cathodes and noble-metal coated titanium hydride anodes

ABSTRACT

Electrolytic cells for the electrolysis of aqueous alkali metal chlorides are disclosed. Further disclosed are electrodes for use in combination with said cells, the said electrodes having titanium hydride cathodic members and noble metal coated titanium hydride anodic members. The noble metal coatings on the anodic members are gold, silver, platinum, palladium, iridium, ruthenium, osmium, and rhodium. In one exemplification, the electrolytic cell is of bipolar configuration and has a bipolar electrode of titanium hydride with a titanium hydride cathodic surface, and a noble metal anodic surface.

Unite States atent [191 Dewitt ELECTROLYTIC CELL HNCLUDHNG TITANIUM HYDRIDE CATHODES AND NOBLE-METAL COATED THTANTUM HYDRIDE ANODES Inventor: Bernard J. Dewitt, Akron, Ohio Assig'nee: Nora International Company,

I Panama City, Panama Notice: The portion of the term of this patent subsequent to Oct. 26, 1988, has been disclaimed.

Filed: Feb. 17, 1971 Appl. No.: 115,992

Related US. Application Data Continuation-in-part of Ser. No. 726,865, May 6, 1968, Pat. No. 3,617,462.

U.S.'Cl. ..204/268, 204/95, 204/98,

204/99, 204/256, 204/290 R, 204/291 lint. Cl. ..B01k 3/06 Field of Search ..204/268, 290 R, 290 F,

[ 51 *May 3,1973

[56] References Cited UNITED STATES PATENTS 3,410,784 11/1968 Maunsell et a] ..204/290 R Primary Examiner-John l-l. Mack Assistant Examiner-W. I. Solomon Attorney-Chisholm and Spencer [57] ABSTRACT Electrolytic cells for the electrolysis of aqueous alkali metal chlorides are disclosed. Further disclosed are electrodes for use in combination with said cells, the said electrodes having titanium hydride cathodic members and noble metal coated titanium hydride anodic members. The noble metal coatings on the anodic members are gold, silver, platinum, palladium, iridium, ruthenium, osmium, and rhodium. In one exemplification, the electrolytic cell is of bipolar configuration and has a bipolar electrode of titanium hydride with a titanium hydride cathodic surface, and a noble metal anodic surface.

12 Claims, 2 Drawing Figures ELECTROLYTIC CELL INCLUDING TITANIUM IIYDRIDE CATIIODES AND NOBLE-METAL COATED TITANIUM I-IYDRIDE ANODES CROSS REFERENCE TO RELATED APPLICATIONS BACKGROUND or THE INVENTION Titanium anodes having noble metal surfaces, platinum surfaces in particular; have achieved some notoriety in recent years in the electrolytic alkali chlorine and chlorate cell fields. This interest and the research and development efforts expended on such anodes is manifested in the many patents issued on such anodes. Thus, U.S. Pat. No. 3,291,714 discloses the use of such anodes in both alkali metal chlorate cells and in alkali metal chlorine cells. Bipolar electrodes as well as monopolar electrodes are disclosed having noble metal coated anodic surfaces. The use of such electrodes in monopolar cell operations is described in U.S. Pat. No. 3,055,821. The use of a platinum coatedtitanium anode for use in an alkali metal chlorine cell having a flowing mercury cathode is described in US. Pat. No. 3,271,289.

Despite the widespread interest in noble metal coated electrodes for use in alkali metal chlorine and chlorate electrolytic cells, they have not achieved any notable success in terms of being utilized on a commercial scale to any appreciable degree. One difficulty frequently attending their use as bipolar electrodes in chlorate cells for example, is the fact that the titanium substrate is found to swell frequently. This swelling of the substrate or titanium base'causes deterioration of the titanium forming the body of the electrode and sloughing off of particles of the titanium electrode which result in contamination of the cell liquor. The platinum coating is also affected by any substantial swelling or distortion of the titanium base thus causing it to lose its adherence to the titanium substrate. Particles of platinum also are lost with particles of titanium as they fall from a swollen electrode into the cell electrolyte.

THE INVENTION In accordance with this invention noble metal coated electrodes are provided having a base of titanium hydride. These electrodes may be employed as bipolar electrodes in an electrolytic cell wherein the noble metal surface is operated as an anodic surface and a titanium hydride surface is operated as a cathodic surface. In a further embodiment of the invention it has been discovered that effective and efficient alkali metal chlorine and chlorate cell anodes can be provided utilizing an anode of titanium hydride having a noble metal surface. In a further embodiment of the invention it has been found that titanium hydride may be successfully employed as a cathodic surface in the operation of alkali metal chlorine and chlorate cells without experiencing any serious electrical or chemical problems.

Bipolar electrolytic cells for the electrolysis of aqueous solutions of alkali metal chlorides have a configuration substantially as described in US. Pat. No. 3,337,443 to Carl W. Raetzsch et 'al. for ELEC- TROLYTIC CELL. Such bipolar electrolytic cells are characterized by a plurality of anodes and cathodes electrically in series and mechanically connected, i. e., in bipolar configuration, whereby each intermediate 0 cathode is electrically in series with the anodes prior and subsequent thereto and, further, each intermediate cathode is mechanically connected to the anode subsequent thereto in the electrolytic cell. Such bipolar electrolytic cells further have means for imposing an electromotive force across the bipolar electrolytic cell and between the first cathode in the cell and the last anode in the cell, whereby electrolysis is carried out therebetween.

It has been found in operating electrolytic alkali metal chlorate cells with bipolar electrodes composed of a titanium hydride mass or substrate having at least one noble metal surface that the cell can be operated to produce chlorate at acceptable efficiency and with no swelling of the electrode. In addition, no appreciable loss of titanium hydride particles or noble metal particles from the bipolar electrode to the cell liquor or electrolyte is noted. In addition, titanium hydride has been employed as the cathode in an alkali metal chlorate cell and found to operate effectively. This electrode was also found to exhibit good corrosion resistant properties in service as a chlorate cell cathode. The titanium hydride cathode may also be employed as a cathode in an alkali metal chlorine cell of the diaphragm type. Similarly the utilization of a titanium hydride electrode with a noble metal surface as a bipolar electrode for alkali metal chlorine cell use or as the anode for an alkali metal chlorine cell of the diaphragm type or an alkali metal chlorine cell of the flowing mercury cathode type is contemplated.

In accordance with this invention the electrode body may take any one of several forms and still perform effectively as a bipolar electrode or a monopolar electrode. Thus, when the electrode is employed as a cathode in a chlorine or chlorate cell, it is typically constructed of titanium hydride. While a body of substantially solid titanium hydride is preferable in cathodic service in chlorine and chlorate cells, the cell cathode can be constructed so that only the cathodic surface is titanium hydride. Thus, as a cell cathode a base plate of metal such as steel having a titanium hydride surface affixed thereto may be utilized to provide the cathodic surface.

In operations where a bipolar electrode is desired for use in alkali metal chlorate or chlorine cells, the titanium hydride has at least one electrical surface which carries as a surface a corrosion resistant, electroconductive metal or metal oxide, for example a noble metal or oxide of a noble metal. Preferably the metal is platinum but other noble metals may be utilized. The use of gold and silver is contemplated as is the use of any of the platinum group metals. Thus in addition to platinum, it is within the contemplation of the instant invention to employ as a surface on a titanium hydride base a coating of such metals as ruthenium, rhodium, palladium, osmium, rhenium and iridium.

The terms noble metal coating and noble metal surface utilized in the specification and claims is intended to include the noble metals hereinabove set forth (i. e., platinum, ruthenium, rhodium, palladium, osmium, rhenium, and iridium) in their metallic state, alloys of these noble metals and their oxides when used in relatively thin layers, i. e., from above 5 micro-inches to about 500 micro-inches. Typical of the oxides intended to be embraced by these terms are the oxides disclosed in French Pat. No. 1,479,762. When oxides of noble metals are employed, they may be utilized singly, in combination with other noble metal oxides (e. g., ruthenium oxide-osmium oxide, rhodium oxideiridium oxide, palladium oxide-platinum oxide, ruthenium oxide-iridium oxide, ruthenium oxideplatinum oxide, rhodium oxide-osmium oxide, rhodium oxide-platinum oxide, palladium oxide-osmium oxide, palladium oxide-iridium oxide), in combination with other noble metals (e. g., ruthenium oxide-osmium, ruthenium oxide-iridium, ruthenium oxide-platinum, rhodium oxide-osmium, rhodium oxide-iridium, rhodium oxide-platinum, palladium oxide-osmium, palladium oxide-iridium, palladium oxide-platinum), or as a mixture of at least one noble metal or oxide of a noble metal and at least one oxide of a non-noble metal (e. g., an oxide of titanium, tantalum, niobium, hafnium, zirconium, tungsten, vanadium, silicon, or the like). Examples of various mixtures of oxides which may be employed are disclosed in French Pat. No. 1,479,762. Additionally, the noble metal or noble metal oxide may have dispersed therein or disposed above it an oxide of chromium, manganese, iron, cobalt, nickel, molybdenum, or mixtures thereof, or a spinel as disclosed in commonly assigned, copending application Ser. No. 106,840, filed Jan. 15, 1971, of Paul P. Anthony for ELECTRODES.

In constructing bipolar electrode elements, it is preferred to utilize a solid titanium hydride base and to plate or coat one surface thereof with the desired noble metal. Also within contemplation is the use of electrode bases which are composed of laminates of metals. Thus, an electrically conductive metal plate such as steel may have affixed to it a layer of titanium hydride as one surface. This titanium hydride surface is then coated with the noble metal so that the bipolar electrode has a steel cathodic surface with a noble metal coated anodic surface. Electrodes of this type have the advantage of utilizing small quantities of titanium hydride thus reducing electrode cost.

In utilizing titanium hydride having noble metal surfaces as the anode in an alkali metal chlorine or chlorate cell, it is preferred that the base be constructed of preformed titanium hydride since the anolyte in such service is corrosive to metals such as steel as is the wet cell gas.

The electrodes of the instant invention may be shaped to provide for their use as anodes in various electrolytic alkali metal chlorine and chlorate cells. Thus by providing them in the form of flat plates they can be conveniently adapted for use as anodes in conventional alkali metal chlorine cells such as the Hooker cell, in the filter press type alkali chlorine cells as well as in the conventional flowing mercury cathode cells. Typical of cells of this character are the cells described in U.S. Pat. Nos. 2,447,547; 3,247,090; 2,627,501 and 2,599,363. Similarly the titanium hydride electrodes of the instant invention may be shaped for use in conventional alkali metal chlorate cells such as those shown in U.S. Pat. Nos. 3,055,821 and 3,291,714.

Titanium hydride may be prepared by the methods described in U.S. Pat. Nos. 2,401,326 and 2,425,711. Titanium hydride electrodes may be prepared by subjecting titanium hydride powders to the application of considerable pressure in a mold. Typically the titanium hydride powder is subjected to pressures in a mold of the desired shape of the electrodes. Pressures on the order of 50 tons per square inch or more are applied to the powder filled mold in an atmosphere of hydrogen at temperatures of 600C. or more in a slight vacuum (400 to 600 millimeters of mercury). If desired the titanium hydride powder may be first pressed into the desired shape in a mold at pressures of from about 12 to about tons per square inch. The shaped titanium hydride electrode may then be placed in an oven in a hydrogen atmosphere and sintered at temperatures of from about 600C. to 1,000C. or more. After the sintering operation the electrode may be tested for the titanium hydride (Tilcontent by use of X-ray diffraction analysis. If desired the electrode after analysis may be subjected to hydrogenation in an oven by surrounding the electrode with hydrogen at temperatures of about 600C. to about l,l00C. at pressures of 400 to 600 millimeters of mercury.

Another convenient method of providing the titanium hydride electrodes of the instant invention involves the direct hydrogenation of the shaped electrode. In this instance the shaped electrodes composed of metallic titanium are subjected to temperatures of 1,000C. to l,200C. in a vacuum oven in an atmosphere of hydrogen at pressures of 400 to 600 millimeters of mercury. The temperature of the heated metal bodies is reduced over a long period of time to eliminate cracking caused by rapid cooling while maintaining a hydrogen atmosphere in the oven. Typically the temperatures are reduced at rates of about 50C. per hour once the 1,000 to l,200C. desired temperature is reached. When the electrode has been cooled to room temperature, it is ready for use as an electrode in an alkali chlorine or chlorate cell.

The noble metal coatings are applied to the titanium hydride conveniently by recourse to conventional electroplating techniques. Thus the titanium hydride electrode is immersed in a plating bath containing the desired noble metal and after protecting the hydride surfaces except for the surface on which the coating is to be deposited the unprotected surface is plated by electrolysis of the platinum containing solution in the conventional manner. Typically platinum diamino nitrite solutions containing 5 grams per liter platinum are employed to provide the platinum for deposition on the titanium hydride surface which during the plating operation forms the cathode of the electrolytic cell. The anodes used may be preferably platinum though graphite has also been employed in such baths as the anode of the cell. Palladium may be plated on a titanium hydride electrode in similar fashion typically from a palladium diamino nitrite solution at concentrations of about 5 grams per liter palladium using a palladium anode. In similar fashion other noble metal surfaces may be applied to the titanium hydride substrate of the novel electrodes herein described.

To furtherillustrate the instant invention reference is made to the accompanying drawing in which:

FIG. 1 is a side view of the cell of FIG. 2 in section taken along lines I1 and,

FIG. 2 is an end view in cross section of a bipolar cell utilizing a bipolar titanium hydride electrode having one noble metal surface.

In the cell shown in FIGS. 1 and 2 the cell box 1 is constructed of Plexiglas and provided with a cover member 3 to effectively cover the cell. The cell was provided near the top of the end wall 15 with openings 13 and 13a for removal of electrolyte from the cell. Gas is removed through openings 16 and 17 in cell top 3. The electrolyte was introduced into the cell through openings 12 and 120 provided near the bottom of the end wall 15. The cell 1 is positioned in a beaker 2, provided with a cover 20. An electrode stem connector 11 passes through an opening 14 in cover 20 of the beaker 2, through opening 16 of cell cover 3 and was electrically connected to the cell anode 8. The stem connector was connected to a power source (not shown). On the opposite side of the cell box 1 was a similar stem connector which passed through opening in cover of the beaker 2 and opening 17 in a cell cover 3 and was electrically connected to the cell cathode 9 at one end and to a suitable power source (not shown) at its other extremity.

Intermediate the anode 8 and the cathode 9 of the cell is the bipolar electrode 5. This electrode is circular in shape and is in the form of a washer. The central portion of the electrode 5 is filled with a Plexiglas plug 7 and is held in place in the cell by a Plexiglas frame 4. The titanium hydride electrode 5 has an anode surface platinum or other noble metal surface 6 which is placed facing the cell cathode 9 while the titanium hydride surface of the electrode faces the anode 8 of the cell.

To illustrate the use of the noble metal coated titanium hydride bipolar electrode in the operation of an alkali metal chlorate cell the following examples'were run.

Example I A cell such as the cell shown in FIGS. 1 and 2 was employed to produce alkali metal chlorate by electrolysis. The titanium hydride anode 5 was placed in a plating bath with one side exposed to a plating solution of platinum diamino nitrite containing 5 grams per liter platinum. A platinum coating was applied to the exposed surface at a current density of 5 amps per square foot for a period of 30 minutes. The titanium hydride electrode with the coated surface 6 was then placed in the cell box 1. The anode 8 of the cell was a platinized titanium anode plate and the cathode 9 was a titanium sheet. The anode 8 was spaced one fourth of an inch from the titanium hydride surface of the bipolar electrode 5 and the cathode 9 was spaced one fourth of an inch from the noble metal surface 6 of the bipolar electrode 5. A 300 grams per liter ACS grade sodium chloride was utilized as electrolyte, and the run was conducted in a batch operation. The volume of electrolyte used was 2,500 milliliters and this was placed in beaker 2 filling it to the level indicated at 21 in FIGS. 1 and 2. The cell was operated at a temperature of about 38C. and the pH of the electrolyte was 8. When the cell was actuated, gas lift in the cell drew the electrolyte into the cell through openings 12 and 12a and discharged electrolytes through openings 13 and 13a.

Cell gas left the cell through openings 16 and 17 of cover 3 and left the beaker through openings 14 and 15 in cover 20.

A current of 3 amps was employed in the cell and the bipolar electrode presented in the cell 5.4 square inches of exposed area on each side of the electrode. Cell voltage during the run across the cell was 6.72 at the start of the run and was 6.85 at the end of the run, which run lasted 69 hours. At the end of the run the cell was dismantled and the bipolar electrode was inspected for swelling or other damage. No damage could be detected by visual observation.

Example II The bipolar electrode from the run of Example I was placed back in the cell used in Example I with the electrolyte still in place from the run of Example I. The cell was again started at 3 amps current and the electrolysis continued for hours at temperatures of between 38 to 40C. The electrolyte pH was 8.3. The cell voltage during the run was between 6.95 and 7.16 across the cell. At the end of the run a total of 159 grams of NaClO had been produced, this being the productivity of this run and the run of Example I. The cell electrolyte on visual observation was clear and free of any precipitated particles. The cells bipolar electrode 5 was upon visual observation found to be undamaged.

Example III The cell container 2 of Example I was recharged with 2,500 milliliters 'of ACS grade sodium chloride at a concentration of 300 grams per liter. The platinum coated titanium hydride bipolar electrode employed was the same electrode utilized in Examples I and II. The cell was connected to a source of DC current and operated at 3 amps current, a pH of 7.8 at between 38C. and 40C. The cell was operated under these conditions for a period of 240 hours. The cell voltage during the run was between 6.85 and 7.24 across the cell. During the run 242 grams of sodium chlorate was produced. At the end of the run the cell was shut down and the cell liquor examined for contamination with electrode particles. No precipitation of any kind was observed in the cell and the electrode appeared to be unchanged. 1

Example IV The cell of Example I was again run using as electrolyte a 300 grams per liter concentration of ACS grade sodium chloride. The platinum coated titanium hydride bipolar electrode from the run of Example III was again used. 2,500 milliliters of electrolyte were added to the cell container 2. The current was turned on the cell at 3 amps current flow. Temperature of operation varied between 39C. and 40C. The cell voltage across the cell varied between 7.01 to 7.1 volts. The run continued for a period of hours. At shut down no observable change had taken place in the bipolar electrode.

Example V The cell of Example I was again employed. The cell container 2 was charged with 2,500 milliliters of ACS grade sodium chloride of a 300 grams per liter concentration. The cell was equipped with the platinum coated titanium hydride electrode from the run in Example IV as the bipolar electrode of the cell. The cell was operated at 38C. to 39C. temperature and current of 3 amps. The cell voltage during the run varied from 6.45 to 6.65 across the entire cell. The cell liquor had a pH of 8.0. The run was continued for 438 hours. A total of 477 grams of NaClO was produced during the run. After shut down the cell was examined for evidence of electrode deterioration. None was observed. The bipolar electrode appeared unchanged and no precipitate of any kind was observed in the electrolyte.

In the above examples the cell voltage reported was across the entire cell which, as will be understood by the skilled artisan, because of the bipolar electrode 5, was essentially a two cell unit. Thus the voltage of the individual cells making up the two cell unit tested was approximately one half of the voltage reported.

In addition to the above examples a titanium hydride electrode not having any noble metal surface thereon was tested as a cathode in an alkali metal chlorate cell.

In these tests the titanium hydride electrode was placed in between two platinized titanium anodes and a cell electrolyte of 2,500 milliliters of ACS grade sodium chloride at 300 grams per liter concentration was employed in container 2. Various temperatures were em ployed and cell voltage varied from between 3.75 to 4 volts. Amperage used during the runs was 3.5. The results of the several runs made under these conditions are set forth below in Table 1:

TABLE 1 either the diaphragm or mercury type, these electrodes of titanium hydride carrying a noble metal coating are both effective and stable and not subject to the shortcomings of noble metal coated titanium electrodes as mentioned hereinabove.

While in discussing the placing of coatings of noble metals of titanium hydride surfaces hereinabove conventional electroplating methods have been shown, it is of course to be understood that other methods for applying noble metal coatings can be utilized. Thus, thermal and chemical methods, as well as galvanic methods, can be employed. Typical of other methods which may be employed for plating electrodes with noble metals as metals, alloys or oxides are those described in French Pat. No. 1,479,762.

While the invention has been described above with reference to certain specific examples and illustrative embodiments, it is not intended that it be so limited thereby except insofar as appears in the accompanying claims.

I claim:

1. In an alkali metal chloride electrolytic cell having a plurality of anodes and cathodes electrically in series and mechanically in bipolar configuration whereby each intermediate cathode is electrically in series with the anodes prior and subsequent thereto and mechanically connected to the anodes subsequent thereto in the electrolytic cell and having means for imposing an electromotive force across said cell, the improvement wherein the cathode is titanium hydride.

2. The alkali metal chloride electrolytic cell of claim 1 wherein the anode comprises titanium hydride having u Run number (hours) at NaClO; end grams 10. 5 lll 10.6 254. 5

After Run 1 the electrode was examined and appeared to be in good condition. After Run 2 the electrode showed a slight weight gain apparently due to hydrogenation. A total of 0.04 gram of particles was found in the cell liquor.

The bipolar electrode of Examples 1 through V can be readily employed as a bipolar electrode in chlorine cell service also. As will be readily understood by the skilled art this type of operation would require a modification of the cell shown herein by the insertion of a diaphragm (typically an asbestos diaphragm) between the anodes and cathodes of the cell. Thus, for example, in the cell used in Example I, a diaphragm placed between the electrodes 8 and 5 and one placed between face 6 of the electrode 5 and cathode 9 will effectively convert the cell to a chlorine cell. In such a chlorine cell operation the noble metal coated bipolar titanium hydride electrode is effective and stable.

In such a chlorine cell operation a titanium hydride electrode can be used as a cathode with success. The

use of the hydride of titanium in such chlorine cell service is far superior to the use of the metal titanium in this instance since little or no hydrogenation of the electrode can occur.

Further when used as the anode of a chlorine cell,

a noble metal coating there 51i 3. The alkali metal chloride electrolytic cell of claim 2 wherein the noble metal coating is selected from the group consisting of platinum, ruthenium, rhodium, palladium, osmium, iridium, and oxides thereof.

4. The alkali metal chloride electrolytic cell of claim 2 wherein the said noble metal coating comprises a mixture of at least one noble metal oxide and at least one oxide of a non-noble metal.

5. The alkali metal chloride electrolytic cell of claim 2 wherein the noble metal coating has dispersed therein an oxide of a metal chosen from the group consisting of chromium, manganese, iron, cobalt, nickel, molybdenum, and mixtures thereof.

6. The alkali metal chloride electrolytic cell of claim 2 wherein the said noble metal coating has disposed above it an oxide of a metal chosen from the group 0nsisting of chromium, manganese, iron, cobalt, nickel, molybdenum, and mixtures thereof.

7. In an alkali metal chloride electrolytic cell having an anode, a cathode, and a means for imposing an electromotive force therebetween, the improvement wherein the anode comprises a titanium hydride substrate having an electroconductive, chemically resistant coating thereon.

8. The alkali metal chloride cell of claim 7 wherein the electroconductive, chemically resistant coating comprises a noble metal coating.

9. The alkali metal chloride electrolytic cell of claim 7 wherein the electroconductive, chemically resistant coating is selected from the group consisting of platinum, ruthenium, rhodium, palladium, osmium, iridium, and oxides thereof.

10. The alkali metal chloride electrolytic cell of claim 7 wherein the said electroconductive, chemically resistant coating comprises a mixture of at least one noble metal oxide and at least one oxide of a non-noble metal. 

2. The alkali metal chloride electrolytic cell of claim 1 wherein the anode comprises titanium hydride having a noble metal coating thereon.
 3. The alkali metal chloride electrolytic cell of claim 2 wherein the noble metal coating is selected from the group consisting of platinum, ruthenium, rhodium, palladium, osmium, iridium, and oxides thereof.
 4. The alkali metal chloride electrolytic cell of claim 2 wherein the said noble metal coating comprises a mixture of at least one noble metal oxide and at least one oxide of a non-noble metal.
 5. The alkali metal chloride electrolytic cell of claim 2 wherein the noble metal coating has dispersed therein an oxide of a metal chosen from the group consisting of chromium, manganese, iron, cobalt, nickel, molybdenum, and mixtures thereof.
 6. The alkali metal chloride electrolytic cell of claim 2 wherein the said noble metal coating has disposed above it an oxide of a metal chosen from the group consisting of chromium, manganese, iron, cobalt, nickel, molybdenum, and mixtures thereof.
 7. In an alkali metal chloride electrolytic cell having an anode, a cathode, and a means for imposing an electromotive force therebetween, the improvement wherein the anode comprises a titanium hydride substrate having an electroconductive, chemically resistant coating thereon.
 8. The alkali metal chloride cell of claim 7 wherein the electroconductive, chemically resistant coating comprises a noble metal coating.
 9. The alkali metal chloride electrolytic cell of claim 7 wherein the electroconductive, chemically resistant coating is selected from the group consisting of platinum, ruthenium, rhodium, palladium, osmium, iridium, and oxides thereof.
 10. The alkali metal chloride electrolytic cell of claim 7 wherein the said electroconductive, chemically resistant coating comprises a mixture of at least one noble metal oxide and at least one oxide of a non-noble metal.
 11. The alkali metal chloride electrolytic cell of claim 7 wherein the electroconductive, chemically resistant coating has dispersed therein an oxide of a metal chosen frOm the group consisting of chromium, manganese, iron, cobalt, nickel, molybdenum, and mixtures thereof.
 12. The alkali metal chloride electrolytic cell of claim 7 wherein the electroconductive, chemically resistant coating has disposed above it an oxide of a metal chosen from the group consisting of chromium, manganese, iron, cobalt, nickel, molybdenum, and mixtures thereof. 